1. The Field of the Invention
The present invention relates to a thin flat glass substrate with a thickness of less than 1.5 mm and to a method for manufacturing this sort of thin flat glass substrate.
2. Related Art
Thin flat glass substrates are, among other things, used to make flat display screens, e.g. plasma display panels (PDP), field emission displays (FED), TFT liquid crystal display screens (TFT=thin film transistor), STN-liquid crystal display screens (STN=Super twisted nematic), PALC display screens (PALC=Plasma assisted liquid crystal), EL displays (EL electroluminescent) and the like.
In flat display screens either a thin layer of liquid crystal compound is placed between two glass panels or respective dielectric layers are applied to the front and rear side of the rear and/or front glass panels, from which cells are formed, in which a phosphor is placed, according to the type of display.
It is important that the layer thickness of the liquid crystal layer and/or the thickness of the dielectric layer is maintained exactly so that especially in the case of display screens with comparatively large dimensions no disturbing color adulteration or brightness variations (shadows) occur. Since layer thickness (currently about 30 microns) is always becoming smaller and display screens are always becoming larger, these requirements have attained increasing importance.
Although float glass is excellently suited for display applications because of its fire polished surface, it has not been possible to make display glass with thickness variations of less than 50 μm according to the float process with the currently required large substrate format with edge lengths of above 1800 mm.
The presence of flows in the float bath, which usually comprise melted tin, explains the presence of thickness variations in float glass. These very complex flows are the result of opposing mechanical and thermally induced flows, i.e. the flow dynamics and thermal effects overlap or are superimposed on each other.
A flow in the motion direction of the glass sheet, i.e. a flow of the hot section of the tin bath in the direction of the cold section arises directly under the glass sheet due to the motion of the glass sheet. In the free surface of the tin bath beside the glass sheet a return flow, i.e. a flow in the opposite direction, arises so that the colder tin flows in the direction of the hotter front section of the tin bath. Temperature non-uniformities, which are transferred to the hot forming glass sheet and lead to viscosity non-uniformities, arise because of the mixing of these flows. These viscosity changes can then lead to undesired thickness fluctuations and waviness in the glass sheet. These fluctuations are the more noticeable, the more strongly the glass sheet is drawn out, i.e. the thinner the glass sheet becomes during manufacture.
Attempts have already previously been made to prevent and/or suppress these lateral return flows by building flow barriers, so-called flags, e.g. as described in DE-PS 1771 762 or DE-PS 2146 063. According to DE-PS the return flow is channeled by means of barriers or dam. The return flow formed between the lateral walls of the float tank and the barriers is suppressed or prevented by means of resistance bodies adjustable in their height and immersed in the return flow. DE-PS 2146 063 describes a special bottom structure for a float bath for guiding the underflow of bath liquid at the bottom of the flow bath, which prevents the lateral return flow by means of lateral baffle plates immersed in the flow bath (FIG. 8 of this reference). EP 031 772 B1 describes the arrangement and action of flags in great detail. In this reference it is also shown that these flags can be arranged not only transversely to the feed direction of the glass sheet, but also can be at an angle to it. In JP 2000-313628 a flag is shown, which is arranged substantially under the bath surface. The angle, at which this flag is immersed in the molten metal, can be adjusted as well as the distance between the flag and the glass sheet.
In spite of the improvements in the flat glass manufacturing process up to now it has not been possible to make large-area thin flat glass substrates with a thickness of less than 1.5 mm, which met high quality specifications.